Matrix converters are becoming an increasingly popular way of providing direct power conversion for variable speed drives and power supplies. In its most basic form, a matrix converter consists of an array of switches that are arranged such that any of a number of input lines can be connected to any of a number of output lines in a predetermined configuration. A conventional single-stage matrix converter is shown in FIG. 1. The matrix converter has three input lines labelled I1, I2 and I3 of a three-phase ac voltage supply network, and three output lines labelled O1, O2 and O3 for supplying a three-phase ac output voltage to a load. Nine bi-directional switches S1 to S9 allow any of the input lines I1, I2 and I3 to be connected to any of the output lines O1, O2 and O3.
The switches are implemented using semiconductor devices and can be bi-directional such that the single-stage matrix converter can be used for both motoring and generating applications. A common bi-directional switch implementation uses a back to back series-connected pair of fast recovery diodes (FRDs) and a pair of Insulated Gate Bipolar Transistors (IGBTs) connected together in anti-parallel with the diodes. The IGBTs can be connected together in a common collector (FIG. 2a) or common emitter (FIG. 2b) arrangement. Other solid-state devices such as Metal Oxide Semiconductor Field Effect Transistors (MOSFETs), Integrated Gate Commutated Thyristors (IGCTs), MOS-Controlled Thyristors (MCTs) and Gate Turn Off Thyristors (GTOs) can be used in place of the IGBTs. It is sometimes also convenient to use a semiconductor device having a reverse-blocking capability such as Reverse Blocking-Insulated Gate Bipolar Transistors (RB-IGBT).
The waveform of the three-phase ac output voltage is usually created using pulse width modulation. Pulse width modulation is described in more detail below in connection with the matrix converter of the present invention, but in terms of FIG. 1, it essentially involves connecting each of the output lines O1-O3 to each of the input lines I1-I3 in a particular sequence and for preselected periods of time. The three-phase ac output voltage is therefore made up of parts of the three-phase input voltage. The switches S1-S9 are normally controlled to open and close in accordance with a particular modulation strategy.
One of the major problems with conventional matrix converters is the lack of natural freewheel paths. The operation of the switches must therefore be carefully controlled. If an output line is connected to two or more of the input lines at the same time then this will cause a short circuit and the resulting large current may cause severe damage to the matrix converter, or even destroy it. On the other hand, if an output line is not connected to any of the input lines then there is no path for the inductive current load and this will cause a large over-voltage. It will be appreciated that currently available semiconductor switches cannot be switched on and off with the necessary degree of precision and it is therefore impossible to prevent short and open circuits on the output lines of the matrix converter without using some orderly form of current commutation. One possible four-step commutation strategy is described in Dr Wheeler, P.; Dr Clare, J.; Dr Empringham, L. “Bidirectional Switch Commutation for Matrix Converters” (EPE '99-Lausanne). This problem is addressed in the matrix converter according to the present invention.
It is also possible to use a matrix converter as a rectifier to provide a dc output voltage to a load. U.S. Pat. No. 6,166,930 to Otis Elevator Company describes a matrix converter that is used to connect a three-phase ac input voltage directly to a dc elevator motor. The matrix converter has three input lines for receiving the three-phase ac input voltage and two output lines for supplying the dc output voltage to the dc field windings of the elevator motor. Six bi-directional switches implemented using IGBTs allow any of the three input lines to be connected to either of the output lines and are controlled in sequence using pulse width modulation to provide the required dc voltage waveform at the input terminals of the elevator motor. The elevator motor is used to drive a sheave to which an elevator car and counterweight are roped. The elevator motor is able to operate in a regenerating mode when the elevator car decelerates, when the elevator car is travelling up with a load that is less than the rated load, and when the elevator car is travelling down with a load that is greater than the rated load, for example. The bi-directional switches permit current to flow in either direction from ac to dc when the elevator motor is in normal operation (motoring mode), or from dc to ac in regenerating mode. The bi-directional switches also permit current to flow with either positive or negative dc voltage polarity.
Although conventional matrix converters are normally used as a single-stage power converter, they can also be used to as part of a two-stage power converter. Klumpner, C.; Blaabjerb, F. “Two stage direct power converters: an alternative to the matrix converter” (IEE Seminar, Matrix Converts, 1 Apr. 2003, Austin Court), describes with reference to FIG. 3(b) a two-stage direct power converter which consists of a matrix converter that is used as a rectification stage, and an inversion stage where an array of IGBTs and FRDs are connected together to form a voltage source inverter (VSI). A surge arrester consisting of a diode and capacitor in series extends across the dc-link of the two-stage converter. This is not a freewheel path.